Nowadays, paper and paperboard find various uses as packaging materials, among which kraft packaging paper and linerboard are used most commonly. Hitherto, kraft packaging paper and linerboard have been made mainly from kraft pulp or waste paper containing kraft pulp. Kraft pulp is a pulp which is produced by digesting wood materials by means of a kraft digesting chemical containing sodium sulfide, so that sulfur compounds such as sulfides and thiolignin inevitably remain in the kraft pulp.
On the other hand, the corrugate medium is made by regenerating waste corrugated box or from a pulp which is obtained by digesting wood materials by a sulfite such as sodium sulfite, so that large amounts of sulfite and lignin sulfonate remain in the paper. These sulfur compounds undesirably promote corrosion or rusting of metals, particularly iron.
In making paper from the thus obtained pulp, various strenghthening agents and sizing agents are added to improve the strength and to adjust the workability of the paper. Usually, aluminum sulfate is used for fixing these additives to the pulp. Namely, the paper is made in an acidic state of pH value ranging between 4 and 5.5 obtained by adding aluminum sulfate to the pulp slurry. Thus, this known method is referred to also as "acidic paper making process" and the paper thus produced exhibits acidic pH values.
Therefore, when a metallic material is packaged in kraft packaging paper or corrugated box produced by the acidic paper making process from a pulp obtained by digesting raw material by means of a digesting solution containing sulfur compounds, the metallic material is undesirably rusted or corroded due to contact with the paper or paperboard, so that the appearance and the function of the packaged metallic material deteriorate to the point of seriously degrading the commercial value of the same.
As explained heretofore, there are many factors which cause corrosion of a metallic material. It has also been shown that the rusting of metallic materials is promoted by bacteria or microorganisms which grow on the packaging paper when the same is placed for a long time under a high humidity. The bacteria tend to appear and grow particularly when the adhesive used in the production of a corrugated board is of the starch type, such as the Steinhall starch paste or the starch paste produced by mixing a high amylose starch and a wet strength agent.
It is often experienced that microorganisms breed on paper produced from a pulp prepared from a material containing waste paper. The breeding of the microorganisms or putrefaction causes a reduction in the pH value of the paper and, in addition, the organic matters are decomposed by the enzymes produced by the microorganisms to generate CO.sub.2, NH.sub.3, H.sub.2 S and H.sub.2 SO.sub.4 to form a so-called corrosive environment. In some cases, a shortage of hydrogen is caused by the presence of anaerobic bacteria so that a part of the packaged material constitutes a cathode which undesirably promotes the corrosion. Similarly, a shortage of oxygen tends to be caused by the presence of aerobic bacteria so that a part of the packaged material serves as an anode to accelerate the corrosion unfavourably.
Hitherto, various countermeasures have been taken for preventing rusting of the packaged material. For instance, it is known to apply a vapor phase inhibitor on the paper. This countermeasure, however, cannot prevent rusting satisfactorily particularly when the metallic material together with the packaging material is placed for a long time under a high humidity.
Under these circumstances, there is an increasing demand for the development of packaging paper for metallic material, improved to satisfactorily prevent rusting of the packaged article even when the same is held for a long time under high humidity.
On the other hand, sulfur-free pulping methods have been proposed. For instance, Japanese Patent Laid-Open No. 47601/1974 discloses a method in which the material is processed by sodium hydroxide and sodium carbonate, while Japanese Patent Laid-Open No. 98988/1980 discloses a method in which wooden fibrous material is processed by a process similar to that shown in Laid-Open No. 47601/1974 to become reverse-flow type pulp. These methods, however, are aimed at preventing pollution or preventing coloring of the paper. Namely, these prior art references do not show nor suggest any metallic material packaging paper or material to which this invention pertains, improved to prevent rusting of the content, i.e. the packaged metallic materials.
Paper making in non-acidic or neutral condition is also known per se. However, such known neutral paper making process has been developed with the goal of preventing color degradation of the paper, particularly paper for printing, and is not intended for use in the manufacture of packaging papers for packaging metallic materials.